The PokéDex on the site isn't saying that Kanto Pokémon will be included, but more so it is just showing them as they are in the Pokédex.
However, it wouldn't come as a surprise that after the National Dex is obtained within the game, rematches against the Gyms and Elites may be called for. This is when it may be possible that the Leaders and Elites will have a revamp in their team with Sinnoh and Hoenn Pokémon included along with stronger levels, but we'll see.
